An earth observation satellite or a space observation satellite that is used on an orbit of a satellite is exposed to a rapid and high change in input and output of heat, which results from environmental change of days and nights due to rays of the sun and shadow of the earth under vacuum environments.
Therefore, an optical mirror mounted to the satellite is also exposed to the change in input and output of heat. When the optical mirror is made of a material having a coefficient of thermal expansion which is not zero, thermal strain is caused to the optical mirror due to the change in input and output of heat. In this case, the change in input and output of heat is not applied to the entire optical mirror but there is a distribution in the change in input and output of heat, and thus, a temperature distribution (temperature gradient) is necessarily generated in the optical mirror. As a result, the optical mirror is not uniformly deformed but local thermal strain is caused corresponding to the temperature distribution. Accordingly, a reflective surface of the optical mirror is deformed due to the thermal strain, resulting in deterioration of optical characteristics such as blurring or distortion of a captured image.
In view of the above-described problem, a method has been suggested in which a heat conduction member is connected, between a mirror and an optical mount which maintains the mirror via a support member, using high heat conduction adhesive, and heat generated from a mirror surface of the mirror is promptly conducted to the optical mount through the heat conduction member to decrease a temperature distribution of the mirror (for example, JP-A 2004-13010 (page 3 and FIG. 1)).
Another method has been suggested in which temperature distribution detection means for detecting a surface temperature distribution on a surface of a mirror and temperature control means including a hot wire heater and a fan for partially heating and cooling a backside of the mirror are provided and the temperature control means is operated based on the detected surface temperature distribution, thereby adjusting the surface temperature distribution of the mirror (for example, JP-A 10-284390 (page 3 and FIG. 3)).
However, according to the above method in which the heat conduction member is connected between the mirror and the optical mount and the heat generated at a mirror surface of the mirror is promptly conducted to the optical mount through the heat conduction member to decrease a temperature distribution of the mirror, the coefficient of thermal expansion of the member configuring the mirror is different from the coefficient of thermal expansion of the high heat conduction adhesion or the heat conduction member. Therefore, thermal strain is inevitably generated at a part to which the heat conduction member is connected, so that local deformation of the mirror at the corresponding part can not be prevented.
In addition, according to the above method of operating the hot wire heater and the fan based on the surface temperature distribution of the mirror and thereby adjusting the surface temperature distribution of the mirror, the coefficient of thermal expansion of the member configuring the mirror is different from the coefficient of thermal expansion of the hot wire heater. Therefore, generation of thermal strain at a part to which the hot wire heater is adhered cannot be prevented, and thus, local deformation of the mirror at the corresponding part can not be prevented.